The present invention relates to a method and an apparatus for obtaining redundancy in a multi-carrier CDMA (Code Division Multiplex Access) system and in particular to a method and a system which is capable of reducing the ratio between the peak signal and the average signal in such a system, the so called Crest factor.
In radio transmission systems, power amplifiers are used in the base stations in order to obtain a sufficiently high output power. The power amplifiers used in conventional radio base stations meet requirements such as high frequency range and high linearity. However, the efficiency of conventional power amplifiers is poor.
Furthermore, it can be shown that if the ratio between the peak signal and the average signal is low, the efficiency of an output amplifier increases. The ratio between the peak signal and the average signal is commonly referred to as the Crest factor. It is therefore favourable that the Crest factor is kept as low as possible, while maintaining performance at a required level.
In CDMA (Code Division Multiplex Access) systems, several spreading channels, which overlap both in time and frequency, are added on top of each other, thereby forming a CDMA carrier. The CDMA carrier is then transmitted over an air interface to mobile telephones or other terminals.
In some CDMA systems, several CDMA carriers are amplified by a single power amplifier, a so called Multi Carrier Power Amplifier (MCPA). Furthermore, the spreading codes of the CDMA system behave in a random manner and therefore the peak signal of a CDMA carrier becomes high. The peak signal is the signal formed by all sub-signal vectors pointing in the same direction and is therefore equal to the peak signal of each sub-signal multiplied by the number of sub-signals. The peak-to-average signal ratio becomes even higher when several carriers are amplified by the same power amplifier (MCPA). This is not desired since the Crest factor then becomes higher.
In order to solve this problem, the amplitude of each carrier may be limited by clipping the signal before the channel shaping filter. The clipping procedure may be carried out in the analogue or the digital domain. Since the clipping is carried out before the channel shaping filter, no spectral widening will occur, which otherwise would interfere with signal in neighbouring channels.
Another way of limiting the peak signal of the CDMA system is to clip-the sum of the base-band signal vectors corresponding to each CDMA carrier. Thereupon the different carriers are separated, filtered in a channel shaping filter, modulated and amplified by a multi carrier power amplifier (MCPA).
Furthermore, European patent application No. EP 0 849 905 discloses a CDMA multi code transmitter, wherein spread spectrum base band signals are summed and transmitted. The power consumption is reduced by delaying signals before the baseband signals are summed, so that the transmission timing of the different baseband signals are shifted with respect to each other.
However, none of the techniques above is capable of providing redundancy in the clipping procedure. Also, the techniques according to the above do not provide optimal performance, since the clipping is performed in the base band, and do not take into account the different phases of different CDMA carriers, when these are to be amplified by the same power amplifier.
It is an object of the present invention to increase the reliability of a CDMA system and at the same time reduce the Crest factor of the system.
This object is obtained by a method and an apparatus wherein clipping for each CDMA carrier is performed on separate circuit boards in a CDMA transmitter, and where the clipping circuit has access to information regarding all carriers to be amplified by the same amplifier.
Thus, since each carrier is clipped separately from all other carriers fed to the same amplifier, but having knowledge of the other carriers, a malfunction of the circuit board comprising the clipping circuit and/or clipping evaluation logic will only affect that particular carrier and the other carriers can still be transmitted correctly.
Furthermore, clipping is carried out for each carrier separately and taking into account information regarding the other carriers to be amplified by the same power amplifier. The clipping procedure can therefore take phase information into account when determining the clipping factor. Therefore, an optimum Crest factor can be obtained for the composite CDMA signal transmitted via the common multi carrier power amplifier (MCPA).